The present invention relates to variable speed controls for electric motors; and more particularly to such controls for regulating the speed of hand-held power tools which are driven by an electric motor.
Hand-held power tools, such as electric drills and dry-wall screwdrivers, utilize an electric motor to power a bit which either drills a hole or turns a screw. Such power tools often incorporate a trigger which is manually operated by the user of the tool with the speed of the motor being controlled by the degree to which the user presses the trigger. This allows the speed of the drill or the screw bit to be varied depending upon the particular application for the tool. For example, the speed of a drill bit can be controlled to correspond to the hardness of the material being drilled.
FIG. 1 shows a typical circuit used in previous hand-held power tools to control the speed of an electric motor 11. When switch 12 closes, capacitor 13 charges at an adjustable rate determined by the position of variable resistor 14 which is controlled by the user. When the voltage across capacitor 13 reaches a threshold potential of diac 15, that device fires becoming conductive and applying a trigger potential to the gate electrode G of a triac 16. When triggered, the triac 16 becomes conductive applying electricity from source 18 to the motor 11. Depending upon the charge rate of capacitor 13, the triac 16 turns on at different phase angles during each half-cycle of the alternating current supplied by source 18. The sooner the triac turns on during each half-cycle, the greater the magnitude of current applied to the motor 11 and thus the faster the motor turns.
This basic motor speed control circuit 10 has a drawback with respect to its use in hand-held power tools. Assuming that the power tool has been turned off for a relatively long time, the capacitor 13 will have been fully discharged due to current leakage through the diac 15 and triac 16. In addition, the capacitor 13 has intrinsic resistance which provides a current leakage path. As a result, the capacitor 13 will begin charging from zero volts when the switch is closed and continues to charge until reaching the firing potential of the diac 15. When conventional diacs fire, their conduction characteristic is such that all of the voltage across the capacitor 13 is not discharged. For example, a typical diac fires at 30-40 volts and once conductive, the voltage across the capacitor 13 drops to 20 volts where it remains through the rest of that half-cycle of the alternating supply voltage.
When the polarity of the alternating supply voltage reverses during the next half-cycle, the capacitor does not begin charging from zero volts, but rather from the residual voltage of opposite polarity from the previous half-cycle. For example, at the end of a positive half-cycle, a positive 20 volts remains across the capacitor 13. When the supply voltage polarity reverses during a negative half-cycle, the residual positive 20 volt charge on the capacitor must first be overcome before the capacitor can charge a negative voltage level at which the diac 15 fires. Thus, after the first half-cycle, a longer time is required between the zero crossing of the alternating supply voltage from source 18 and the triggering of the triac 16.
Because the initial current through the motor may be significantly greater than desired or expected by the user, the speed of the motor may increase abruptly when first turned on. Such an abrupt jump in speed can cause the power tool to "kick" in the user's hand. When the tool kicks, the drill bit may move away from the desired location for the hole or the screwdriver bit may jump out of the grooves in the head of the screw.
A similar kick can occur when the power tool is rapidly cycled off and on. As noted previously, current leakage in various components when the power tool is turned off slowly discharges any voltage remaining across capacitor 13. Normally, this leakage current is very small and the discharge occurs over a relatively long period of time. Therefore, rapid cycling of the power tool between off and on, as frequently occurs with a power tool used to drive dry-wall screws, can take place before residual voltage across capacitor 13 has discharged completely.
The amount of that residual voltage is dependent upon the point in the half-cycle of the alternating supply voltage at which the switch 12 was opened. In the worst case situation, the switch 12 is opened just before the voltage across the capacitor 13 has reached the firing potential of diac 15. In this situation if the switch 12 then is closed a very short time later during a half-cycle having the same polarity as the half-cycle when the switch opened, a very short amount of time will elapse before the voltage across capacitor 13 reaches the firing potential of the diac. Thus, the diac may fire almost immediately upon closure of switch 12 causing the triac 16 to apply a relatively large magnitude of current through motor 11 during that half-cycle, even though the user has placed variable resistor 14 in a position which should normally apply a much smaller magnitude of current to the motor. During subsequent half-cycles of the supply voltage that smaller intended current level will be applied. Thus, the motor possibly can start at a relatively high initial torque before it subsequently is powered at a desired speed during the next half-cycle of the supply voltage. Such operation in this worst case creates a very abrupt change in the torque of the motor which also causes jumping of the drill bit or screwdriver bit. This abrupt change in the applied motor current is very undesirable in hand-held power tools.
Another problem often encountered in thyristor based, full-wave phase control circuits is called "hysteresis" wherein the power tool motor stalls or turns off at a different setting of the variable resistor 14 than the setting at which the motor initially turned on. This phenomenon is well known and is described in detail at pages 252-255 of the book entitled SCR Manual Including Triacs and Other Thyristors, 1979 published by the General Electric Company and Prentice-Hall, Inc. Thus once the motor starts the user is able to reduce its speed by backing off the setting of the variable resistor. This phenomenon is commonly referred to as "snap-on" since a single time constant phase control circuit usually has to be started at a higher phase conduction angle that it can run at once started. The snap-on operation is disadvantageous since the motor must start at a higher speed than perhaps is desired by the user. In the past, snap-on was cured by a dual time constant control circuit which also suffered from the motor kick problem.
The power tool market is very cost-competitive and sophisticated electronic circuit solutions to these problems may have an adverse impact on the cost of the power tool. Therefore, it is highly desirable to solve the problems with a minimal number of additional components that are relatively inexpensive.